60 GENETICS IN RELATION TO AGRICULTURE 



results from the longitudinal splitting of the original thread. Both parts 

 of the thread are quantitatively and qualitatively equal, for, by the 

 splitting of all the chromomeres both of the threads come to possess 

 all of the individual elements of the original spireme thread. Following 

 the splitting of the chromomeres and the formation of a double spireme, 

 the spireme-thread contracts and segments transversely forming four 

 double chromosomes, the number characteristic of the cells of this 

 individual. This is the stage shown at C where also is shown the origin 

 of the spindle, a part of the mechanism in mitosis. The chromosomes 

 now still further contract until they assume their characteristic shapes 

 and sizes. They next appear in an equatorial position on the spindle as 

 shown at D, where the two pairs of double chromosomes, one larger 

 and one smaller, are diagrammed and the nucleolus, the large black 

 body of the previous steps, is shown cast out and degenerating. The 

 daughter chromosomes of each pair now separate from each other 

 until at E they have moved nearly to the opposite poles of the spindle 

 and are beginning to fray out and seemingly to lose their identity. At 

 this stage actual division of the cell body has begun. Finally at F, the 

 chromosomes have completely lost all appearance of their identity, the 

 chromatin material is distributed throughout the nucleus as in the origi- 

 nal cell shown at A, and the nucleolus has been reformed in each nucleus. 

 Division of the cell-body has resulted in two daughter cells each of which, 

 so far as chromomeres are concerned, contains exactly the same chromatin 

 elements as the original cell. 



There are many variations in this process particularly in the order of 

 occurrence of the steps, but these variations in nowise modify the essen- 

 tial fact of mitosis which is that the chromatin material of the cell is 

 converted into a thread which splits throughout its entire length into two 

 halves so that the daughter nuclei receive exactly equivalent portions 

 of chromatin material. This precise division of the chromatin is brought 

 about by a division of each chromomere so that not only do the daughter 

 nuclei receive equivalent portions of chromatin but these portions are 

 also equivalent qualitatively to the entire chromatin content of the 

 mother cell. By this method then each of the cells of the body finally 

 comes to possess not only the whole number of chromosomes contrib- 

 uted by the two parents, but also the entire set of chromatin elements 

 which it received from them. The extreme care with which the cell 

 mechanism partitions the chromatin material in each successive cell 

 division is in itself eloquent testimony of the fundamental importance 

 of this material. 



The Production of Germ Cells. In the production of germ cells a 

 different set of phenomena occur which result in a reduction of this num- 

 ber of chromosomes to one-half that characteristic of the somatic cells. 



